Multi-output switching power source circuit

ABSTRACT

A multi-output switching power supply circuit easily produces multiple outputs with increased power source conversion efficiency. The circuit includes, in place of the rectifying diode and the commutating diode used in the multi-output switching power supply circuit of the prior art, a circuit configuration in which an NMOS for synchronous rectification is combined with a constant-voltage control by a magnetic amplifier. It is not required to use, for example, a radiator to dissipate heat, and hence the system size is reduced and the conversion efficiency is increased, and the system can be easily implemented in a low-voltage multi-output configuration. In a configuration in which a magnetic amplifier is arranged between a secondary winding and an first NMOS for synchronous rectification and a drive circuit for the first NMOS and an second NMOS for synchronous rectification is implemented as a separate winding other than the secondary winding, the first NMOS is not included in a loop to flow a reset current. The magnetic amplifier can conduct constant-voltage control without any influence from the interruption of the control loop when the first NMOS on the rectifying side is turned off.

BACKGROUND OF THE INVENTION

The present invention relates to a multi-output switching power sourcecircuit to conduct constant-voltage control using a magnetic amplifieror a magamp.

Description of the Prior Art

FIG. 1 shows a conventional circuit configuration of a multi-outputswitching power supply circuit to conduct constant-voltage control usinga magnetic amplifier or a transducer.

The power supply 20 includes a transformer 20 including a primary sidewhich includes a direct-current (dc) power source 1, an input smoothingcondenser or capacitor 2, a starting resistor 3, a pulse widthmodulation controller 4, detecting resistors 5 and 6, a capacitor 7, asmoothing choke coil 8, a rectifying diode 9, a commutating diode 10,and a main switch 11, e.g., an n-type metal-oxide semiconductortransistor (to be simply referred to as an NMOS hereinbelow).

On the primary side of the transformer 20, a primary winding 21 and anauxiliary winding 22 are disposed. The transformer 20 includes asecondary side including secondary windings 23 a, 23 b, 23 c, etc. forrespective output sections A, B, C, etc., respectively.

The output section A includes a magnetic amplifier 31, a rectifier diode32, a commutator diode 33, a smoothing choke coil 34, a capacitor 35, adummy resistor 36, a constant-voltage control circuit 37, detectingresistors 38 and 39, a transistor 40, a resistor 41, and a diode 42. Theother output sections B, C, and the like are configured substantially inthe same manner as for the output section A. Each section includes aload RL in its output section.

Referring next to FIGS. 2 and 3, description will be given of aprinciple of operation of the magnetic amplifier shown in FIG. 1.

As can be seen from a graph of FIG. 2, the magnetic amplifier 31 is onwhen a pulse current having a pulse width of x μs (micro sec.) isflowing in the circuit. Even when the pulse current repeatedly changesits state between an on state and an off state, the magnetic amplifier31 is in a magnetized state which conducts reciprocation between point Acorresponding to a maximum value of the pulse current and point Bcorresponding to a state in which the current or a magnetic fieldassociated therewith is zero as shown in FIG. 2. The magnetic amplifier31 is kept retained in the on state. However, when a current slightlyflows through the amplifier 31 in a direction opposite to that of thepulse current, that is, when a reset current flows therethrough, thestate of magnetization of the amplifier 31 changes to a statecorresponding to point C. The amplifier 31 therefore turns off. In thissituation, even when voltage E is applied to the amplifier 31 in aforward direction, the current does not flows at once. According to arelationship

Magnetic flux (φ)=Product of Voltage and Time (T×E),

the current starts flowing with a delay of time, i.e., rising time of

ΔT=Δφ/E.

By controlling the rising time delay ΔT by the reset current, the pulsewidth modulation is carried out. In this case, if

x=ΔT,

no current flows at all. In other words, by regulating the width of Δφof the amplifier 31, the pulse modulation is conducted in a range ofpulse current from 0% to 100%.

Subsequently, description will be given of operation of a multi-outputswitching power source circuit of the prior art shown in FIG. 1.

In the power supply circuit, the dc power source section 1 generates adc input voltage V1. The input smoothing capacitor 2 smoothes thevoltage V1.

The PWM (power width modulation) control circuit 4 produces a controlsignal V4 having a predetermined frequency and a pulse widthcorresponding to detected voltage, which is detected as below. Theauxiliary winding 22 on the primary side of the transformer 20 generatesan alternating-current (ac) voltage. The rectifying diode 9 rectifiesthe ac voltage into a pulsating voltage. The smoothing choke coil 8 andthe smoothing capacitor 7 smooth the pulsating voltage to obtain anoutput dc voltage. The resistors 5 and 6 divides the dc voltage. The PWMcontrol circuit 4 detects a change in the divided voltage to therebyproduce the detected voltage. The secondary winding 23 a produces an acvoltage determined by a turn ratio, i.e., a ratio between a number ofturns of the primary winding 21 and that of the secondary winding 23 a.By producing an ac voltage proportional to the ac voltage in thesecondary winding 23 a by the auxiliary winding 22, the PWM controlcircuit 4 controls the pulse width according to the change in the acvoltage to resultantly keep the output voltage at a fixed value. TheNMOS 11 turns on or off the input dc voltage V1 according to the controlsignal V4 to generate an ac voltage V11 having a predetermined frequencyand a pulse width associated with the detected voltage. The transformer20 transforms the ac voltage V11 to produce ac voltages V23 a, V23 b,V23 c, etc. respectively from the secondary windings 23 a, 23 b, 23 c,etc. according to turn ratios respectively between the primary andsecondary windings.

The magnetic amplifier 31 converts the ac voltage V23 a through on/offcontrol using a reset current into an ac voltage V31 having a pulsewidth associated with the reset current. The rectifier diode 32rectifies the ac voltage V31 to produce a pulsating voltage V32. Thevoltage V32 has electromagnetic energy of, which is accumulated in thesmoothing choke coil 34. When the diode 32 on the rectifying side is offand the diode 33 on the commutating side is on, the electromagneticenergy is supplied to the smoothing capacitor 35. The capacitor 35smoothes the pulsating voltage V32 into an output do voltage. The outputsection A feeds the do voltage V to the load RL.

The magnetic amplifier 31 stabilizes the dc voltage using a hysteresischaracteristic. That is, the resistors 38 and 39 detects variation inthe dc output voltage. The constant-voltage control circuit 37 adjuststhe reset current 142 for the magnetic amplifier 31 to stabilize the dcvoltage. During a period in which the amplifier 31 is off, the adjustedreset current 142 is delivered via the transistor 40, the resistor 41,and the diode 42 to the amplifier 31. This resultantly controls therising edge of a period in which the amplifier 31 is on to therebystabilize the de output voltage.

Referring next to FIG. 4, description will be given of a circuitconfiguration of a second example of the multi-output switching powersupply circuit of the prior art using a magnetic amplifier to control aconstant voltage.

The multi-output switching power source circuit includes a main outputsection A and a plurality of subsidiary output sections B, C, etc. Amongthe output sections, the main output section A has a maximum output andsmall load variation. A switching duty ratio on the primary side iscontrolled by a negative feedback operation according to variation in anoutput voltage from the main section A. Each of the subsidiary outputsections produces an output voltage. For the output voltage, themagnetic amplifier controls and produces an ac voltage having a dutyratio determined according to the output voltage from the main outputsection A.

The multi-output switching power supply circuit of the conventionalexample 2 shown in FIG. 4 includes, on the primary side of a voltagetransformer 60, a de power source 51, an input smoothing capacitor 52, astarting resistor 53, a PWM control circuit 54, a capacitor 55, asmoothing choke coil 56, a rectifying diode 57, a commutating diode 58,and an NMOS 59.

The transformer 60 includes a primary winding 61 and a subordinatewinding 62 on the primary side and secondary windings 63, 64, 65, etc.on its secondary side.

The main output section A includes a rectifying diode 71, a commutatingdiode 72, a smoothing choke coil 73, a smoothing capacitor 74, a dummyresistor 75, and a constant-voltage control circuit 76. The main outputsection A is connected to a load RL1. The subordinate section B includesa magnetic amplifier 79, a rectifying diode 80, a commutating diode 81,a smoothing choke coil 82, a smoothing capacitor 83, a constant-voltagecontrol circuit 84, resistors 85 and 86, a transistor 87, a resistor 88,and a diode 89. The subordinate output section B is connected to a loadRL2. The subordinate output section C is configured substantially in thesame way as for the subordinate output section C and is connected to aload RL3.

The secondary winding 63 of the transformer 60 produces an ac voltage.The rectifying diode 71 rectifies the ac voltage into a pulsatingvoltage V71 having electro-magnetic energy. The smoothing choke coil 73accumulates the electro-magnetic energy. When the rectifying diode 71 isoff and the commutating diode 72 is on, the electro-magnetic energy isfed to the smoothing capacitor 74. The capacitor 74 smoothes thepulsating voltage V71 into a dc output voltage V1 to be applied to thedummy resistor 75 and the load RL1. When the output voltage V1 changes,the constant-voltage control circuit 76 detects the voltage change toproduce a detection signal V76. The signal V76 is supplied to the PWMcontroller 54, which conducts negative feedback control for a pulsewidth of an ac voltage V59.

According to a duty ratio determined by the PWM controller 54, thesecondary winding 64 of the transformer 60 generates an ac voltage V64corresponding to a turn ratio between the primary winding 61 and thesecondary winding 64. The ac voltage V64 is fed via a magnetic amplifier79 of the subordinate output section B to be rectified by a diode 80into a pulsating voltage V80. The voltage V80 has electro-magneticenergy, which is accumulated in the smoothing choke coil 82. When therectifying diode 80 is off and the commutating diode 81 is on, theelectro-magnetic energy is supplied to the smoothing capacitor 83. Thesmoothing capacitor 83 smoothes the pulsating voltage V80 into a deoutput voltage V2. The subordinate output section B feeds the dc outputvoltage V2 to the load RL2. The resistors 85 and 86 detects variation inthe voltage V2, and the constant-voltage controller 84 accordinglyadjusts a reset current 189 for the magnetic amplifier 79 to stabilizethe dc output voltage V2. When the NMOS 59 is off, that is, when therectifying diode 80 is off, the reset current is delivered via thetransistor 87, the resistor 88, and the diode 89 to the magneticamplifier 79. As a result, the rising time of the on period of themagnetic amplifier 79 is controlled to stabilize the dc output voltageV2. The subordinate output section C operates in almost the same way asthe subordinate output section B.

The Japanese Patent No. 2927734 describes a low-loss output circuit,which is conventional example 3 associated with the technical field ofthe present invention. As shown in FIG. 5, the prior art is a low-lossoutput circuit including a magnetic amplifier MA connected to asecondary winding N2 of a voltage transformer producing an ac voltagehaving a rectangular waveform, a rectifying element Q1 including an MOSfield-effect transistor (FET) on a rectifying side, a rectifying elementQ2 including an MOS-FET on a flywheel side, and a smoothing choke coilCH and a smoothing capacitor C which smooth outputs from the rectifyingelements to produce a dc output voltage. The smoothing choke coil CHsupplies a signal to drive the rectifier element Q2.

In the configuration, the smoothing choke coil CH to smooth the outputfrom the rectifier element including an MOS-FET delivers a drivingsignal to the rectifying element Q2 on the flywheel side to turn theMOS-FET Q2 on. Therefore, the driving signal is fed to the MOS-FET onthe smoothing choke coil side to turn the MOS-FET on during a periodfrom when polarity of the secondary winding of the transformer ischanged to when the magnetic amplifier is saturated after a lapse of itspredetermined controlled period of time. According to the JapanesePatent Ser. No. 2927734, this resultantly reduces power loss on theflywheel side and hence efficiently lowers the overall loss.

In the multi-output switching power supply circuit in which constantvoltage control is conducted using a magnetic amplifier as above, adiode is generally employed in its rectifying circuit. The use of such adiode in the rectifier circuit leads to a problem that power loss due toa voltage drop in the diode lowers conversion or transformationefficiency.

There also arises a problem as below. Since circuits of large-scaleintegration are operated with a lower voltage as a power source voltagethereof, there are highly required output voltages of +3.3 V, +2.5 V,+1.8V, etc. However, the voltage drop of the diode is almost fixed,about, 0.4 V. This consequently results in a problem. That is, when theoutput voltage of the power supply circuit is reduced, the power loss inthe rectifier circuit including such a diode becomes relatively largerin the overall loss in the power source circuit. This further lowers theconversion loss and hinders the lowering of the output voltage.

The problem of the multi-output switching power source of the prior artwill now be described by referring to the configuration of theconventional example 2 shown in FIG. 4.

Assume that the power source circuit of the prior art does not includethe dummy resistor 75. In this situation, when the load RL1 connected tothe main output section A is reduced, for example, as in a no-load stateand hence a load current thereof becomes equal to or less than acritical current of the smoothing choke coil 73, energy accumulated inthe choke coil 73 is stored in the smoothing capacitor 74 to resultantlyincrease the dc output voltage V1. To suppress the increase in theoutput voltage V1, a control operation is conducted to reduce a timewidth of the on state of the main switch (NMOS) 59. In this situation,the pulse width of the ac voltage generated by the secondary winding 63becomes smaller depending on cases. Therefore, it is impossible toguarantee the period of time or the voltage-time product necessary forthe magnetic amplifier 79 in the subordinate output section B (betweenthe voltage applied across the magnetic amplifier 79 and the timerequired for the saturation of the magnetic amplifier 79). To cope withthe difficulty, a dummy resistor 75 is arranged in the main outputsection A. The resistor 75 keeps the time width of the on state of themain switch (NMOS) 59 to thereby guarantee the voltage-time productnecessary for the magnetic amplifier 79. This leads to a problem thatthe dummy resistor continuously requires power and hence the powerefficiency is lowered. This leads to an additional problem. That is, forthe dummy resistor 75, a radiator is required to cool the dummy resistor75 or an electronic dummy circuit is required, and hence the number ofconstituent components is increased.

Moreover, the low-loss output circuit of the conventional example 3achieving constant voltage control by a magnetic amplifier and includinga synchronous rectifying element using an MOS-FET has an object in whichthe smoothing choke coil supplies a driving signal to the rectifierelement on the flywheel side to prevent a state in which the MOS-FETs Q1and Q2 are on at the same time to thereby suppress occurrence of ashort-circuit current. The invention is therefore not associated withoutput voltage control in the technical field of the present invention.

In addition, no consideration has been given to influence of failure onthe primary side of the voltage transformer upon the secondary sidethereof in the conventional example 3. For example, in the circuitconfiguration of FIG. 6, the voltage transformer includes a core whichis reset by a free resonance caused by inductance of the transformer anddrain-source capacitance of an NMOS 91 including a gate electrode. As aresult, the gate electrode of the NMOS 93 is applied with a flybackvoltage as shown in FIG. 7, and hence its conductive state isdeteriorated.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention, which has beendevised to solve the problems, to provide a multi-output switching powersupply circuit which can increase power source conversion efficiency toeasily increase the number of outputs.

Another object of the present invention is to provide a multi-outputswitching power supply circuit including a main output section andsubordinate output sections in which the subordinate output section canproduce dc output voltages in a stable state without employing a dummyresistor and an electronic dummy resistor in the main output section.

In accordance with a first aspect of the present invention, there isprovided a multi-output switching power supply circuit, comprising a dcpower source for generating a dc input voltage; a detecting circuit fordetecting a voltage value of a second ac voltage generated by a firstsubordinate winding, the first subordinate winding constituting avoltage transformer including a primary side, a primary winding, a core,a secondary side, and a secondary winding; a switching circuit forturning on or off the dc input voltage according to a control signalgenerated by detecting variation in the voltage value of the second acvoltage and thereby producing a first ac voltage having a predeterminedfrequency and a pulse width corresponding to the second ac voltage; acontrol circuit for generating the control signal according to variationin the voltage value of the second ac voltage detected by said detectingcircuit; an active clamp circuit for passing an exciting current throughthe primary winding of said voltage transformer during an off period ofsaid switching circuit and for thereby resetting the core of saidvoltage transformer, said dc power source, said detecting circuit, saidswitching circuit, said control circuit, and said active clamp circuitbeing arranged on the primary side of said voltage transformer; and aplurality of output sections disposed on the secondary side of saidvoltage transformer, each of said output sections comprising a magneticamplifier for controlling, according to a reset current, on or off of athird ac voltage generated on the secondary winding through voltageconversion of the first ac voltage by said voltage transformer and forthereby generating a fourth ac voltage having a pulse widthcorresponding to the reset current; a rectifying circuit for rectifyingthe fourth ac voltage into a pulsating voltage; a smoothing circuit forsmoothing the pulsating voltage into a do output voltage and forapplying the dc output voltage to a load; and a voltage control circuitfor detecting variation in the dc output voltage and for generating thereset current to conduct negative feedback control for the fourth acvoltage. Moreover, said rectifying circuit comprises a first NMOStransistor which is turned on or off according to a voltage value of afifth ac voltage generated on a second subordinate winding disposed onthe secondary side of said voltage transformer and which therebygenerates the pulsating voltage; said smoothing circuit comprises asmoothing capacitor for smoothing the pulsating voltage into the dcoutput voltage and for applying the dc output voltage to a load; a chokecoil for accumulating electromagnetic energy associated with thepulsating voltage; and a second NMOS transistor which turns on, whensaid first NMOS transistor is off, according to a voltage value of asixth ac voltage generated on a third subordinate winding disposed onthe secondary side of said voltage transformer and which therebysupplies the electromagnetic energy from the choke coil to the smoothingcapacitor; and said magnetic amplifier is arranged between the secondarywinding and said first NMOS transistor.

In accordance with a second aspect of the present invention, there isprovided a multi-output switching power supply circuit, comprising a dcpower source for generating a dc input voltage; a detecting circuit fordetecting a voltage value of a second ac voltage generated by a firstsubordinate winding, the first subordinate winding constituting avoltage transformer including a primary side, a primary winding, a core,a secondary side, and a secondary winding; a switching circuit forturning on or off the dc input voltage according to a control signalgenerated by detecting variation in the voltage value of the second acvoltage and thereby producing a first ac voltage having a predeterminedfrequency and a pulse width corresponding to the second ac voltage; acontrol circuit for generating the control signal according to variationin the voltage value of the second ac voltage detected by said detectingcircuit and a level of a detection signal detected by a voltagevariation detecting circuit; an active clamp circuit for passing anexciting current through the primary winding of said voltage transformerduring an off period of said switching circuit and for thereby resettingthe core of said voltage transformer, said dc power source, saiddetecting circuit, said switching circuit, said control circuit, andsaid active clamp circuit being arranged on the primary side of saidvoltage transformer; a main output section disposed on the secondaryside of said voltage transformer, comprising a first rectifying circuitfor rectifying a seventh ac voltage generated on a first secondarywinding through voltage conversion of the first ac voltage by saidvoltage transformer and for thereby generating a first pulsatingvoltage; a first smoothing circuit for smoothing the first pulsatingvoltage into a first dc output voltage and for applying the first dcoutput voltage to a load; and the voltage variation detecting circuitfor detecting variation in the first de output voltage into a detectionsignal and for supplying the detection signal to said control circuit;and a plurality of output sections disposed on the secondary side ofsaid voltage transformer, each of said output sections comprising amagnetic amplifier for controlling, according to a reset current, on oroff of an eighth ac voltage generated on the secondary winding throughvoltage conversion of the first ac voltage by said voltage transformerand for thereby generating a ninth ac voltage having a pulse widthcorresponding to the reset current; a second rectifying circuit forrectifying the ninth ac voltage into a second pulsating voltage; asecond smoothing circuit for smoothing the second pulsating voltage intoa second dc output voltage and for applying the second dc output voltageto a load; and a voltage control circuit for detecting variation in thesecond dc output voltage and for generating the reset current to conductnegative feedback control for the ninth ac voltage. Said firstrectifying circuit comprises a first NMOS transistor which turns theseventh ac voltage on or off at timing synchronized with switchingtiming of said switching circuit and which thereby generates the firstpulsating voltage; said smoothing circuit comprises a first smoothingcapacitor for smoothing the first pulsating voltage into the first dcoutput voltage and for applying the first dc output voltage to a load; afirst choke coil for accumulating electromagnetic energy associated withthe first pulsating voltage; and a second NMOS transistor which turns onwhen said first NMOS transistor is off, and which thereby supplies theelectro-magnetic energy from the choke coil to the smoothing capacitor;said second rectifying circuit comprises a third NMOS transistor whichis turned on or off according to a voltage value of a tenth ac voltagegenerated on the second subordinate winding disposed on the secondaryside of said voltage transformer; said second smoothing circuitcomprises a second smoothing capacitor for smoothing the secondpulsating voltage into the second dc output voltage and for applying thesecond dc output voltage to a load; a second choke coil for accumulatingelectromagnetic energy associated with the second pulsating voltage; anda fourth NMOS transistor which turns on, when said fourth NMOStransistor is off, according to a voltage value of an 11th ac voltagegenerated on a third subordinate winding disposed on the secondary sideof said voltage transformer and which thereby supplies theelectromagnetic energy from the second choke coil to the secondsmoothing capacitor; and said magnetic amplifier is arranged between thesecondary winding and said third NMOS transistor.

In accordance with a third aspect of the present invention, there isprovided a multi-output switching power supply circuit, comprising a dcpower source for generating a dc input voltage; a detecting circuit fordetecting a voltage value of a second ac voltage generated by a firstsubordinate winding, the first subordinate winding constituting avoltage transformer including a primary side, a primary winding, a core,a secondary side, and a secondary winding; a switching circuit forturning on or off the dc input voltage according to a control signalgenerated by detecting variation in the voltage value of the second acvoltage and thereby producing a first ac voltage having a predeterminedfrequency and a pulse width corresponding to the second ac voltage; acontrol circuit for generating the control signal according to variationin the voltage value of the second ac voltage detected by said detectingcircuit and a level of a detection signal detected by a voltagevariation detecting circuit; an active clamp circuit for passing anexciting current through the primary winding of said voltage transformerduring an off period of said switching circuit and for thereby resettingthe core of said voltage transformer, said dc power source, saiddetecting circuit, said switching circuit, said control circuit, andsaid active clamp circuit being arranged on the primary side of saidvoltage transformer; and an output section disposed on the secondaryside of said voltage transformer, comprising a magnetic amplifier forcontrolling, according to a reset current, on or off of a third acvoltage generated on the secondary winding through voltage conversion ofthe first ac voltage by said voltage transformer and for therebygenerating a fourth ac voltage having a pulse width corresponding to thereset current; a rectifying circuit for rectifying the fourth ac voltageinto a pulsating voltage; a smoothing circuit for smoothing thepulsating voltage into a dc output voltage and for applying the dcoutput voltage to a load; and a voltage control circuit for detectingvariation in the dc output voltage and for generating the reset currentto conduct negative feedback control for the fourth ac voltage.Moreover, said rectifying circuit comprises a first NMOS transistorwhich turns the third ac voltage on or off at timing synchronized withswitching timing of said switching circuit and which thereby generatesthe pulsating voltage; said smoothing circuit comprises a smoothingcapacitor for smoothing the pulsating voltage into the dc output voltageand for applying the dc output voltage to a load; a choke coil foraccumulating electromagnetic energy associated with the pulsatingvoltage; and a second NMOS transistor which turns on when said firstNMOS transistor is off, and which thereby supplies the electro magneticenergy from the choke coil to the smoothing capacitor; said first NMOStransistor includes a gate electrode, a source electrode, and a drainelectrode, said gate electrode being connected to a winding end side ofthe secondary winding, said source electrode being linked with a groundside, said drain electrode being coupled with a winding start side ofthe secondary winding; said second NMOS transistor includes a gateelectrode, a source electrode, and a drain electrode, said gateelectrode being connected to the winding start side of the secondarywinding, said source electrode being linked with a ground side, saiddrain electrode being coupled with an output port of said magneticamplifier; said magnetic amplifier is arranged between the gateelectrode of said first NMOS transistor and the drain electrode of saidsecond NMOS transistor; and the reset current is supplied to a windingstart side of said reset winding and is outputted to the ground side.

In accordance with a fourth aspect of the present invention, themulti-output switching power supply circuit of one of the first to thirdaspects described above further comprises a diode having a small voltagedrop in a stage after said second NMOS transistor in parallel with saidsecond NMOS transistor.

In accordance with a fifth aspect of the present invention, in themulti-output switching power supply circuit of one of the first to thirdaspects described above, said active clamp circuit comprises a capacitorconnected to a winding end side of the primary winding of said voltagetransformer; and an NMOS transistor including a gate electrode, a sourceelectrode, and a drain electrode, said gate electrode being connected toa signal delivered from said control circuit, said signal being oppositein phase to the control signal generated from said control circuit; saidsource electrode being coupled with an output from said switchingcircuit, said drain electrode linked with said capacitor, the signalopposite in phase to the control signal generated from said controlcircuit having deadtime preventing an event in which said switchingcircuit and said NMOS are on at the same time.

In accordance with a sixth aspect of the present invention, in themulti-output switching power supply circuit of one of the first to thirdaspects described above, said third ac voltage has a pulse widthnecessary for saturation of said magnetic amplifier.

In accordance with a seventh aspect of the present invention, in themulti-output switching power supply circuit of the second aspectdescribed above, said eighth ac voltage has a pulse width necessary forsaturation of said magnetic amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the present invention will become moreapparent from the consideration of the following detailed descriptiontaken in conjunction with the accompanying drawings in which:

FIG. 1 is a block diagram showing a configuration of a multi-outputswitching power source circuit of the prior art;

FIG. 2 is a graph to explain a function of a magnetic amplifier;

FIG. 3 is a graph to further explain a function of a magnetic amplifier;

FIG. 4 is a block diagram showing a configuration of a multi-outputswitching power source circuit of the prior art;

FIG. 5 is a circuit diagram showing a configuration of a multi-outputswitching power source circuit of the prior art;

FIG. 6 is a block diagram showing a configuration of a multi-outputswitching power source circuit of the prior art;

FIG. 7 is a graph to explain a problem occurring in a multi-outputswitching power source circuit of the prior art;

FIG. 8 is a block diagram showing a configuration of a first embodimentin accordance with the present invention;

FIG. 9 is a graph to explain a function of an active clamp circuit;

FIG. 10 is a graph showing signal waveforms in the first embodiment;

FIG. 11 is a block diagram showing a configuration of a variation of thefirst embodiment in accordance with the present invention;

FIG. 12 is a graph showing signal waveforms in the variation of thefirst embodiment according to the present invention;

FIG. 13 is a block diagram showing a configuration of a secondembodiment in accordance with the present invention; and

FIG. 14 is a block diagram showing a configuration of a third embodimentin accordance with the present invention.

DESCRIPTION OF THE EMBODIMENTS

Referring next to the accompanying drawing, description will be given indetail of embodiments of a multi-output switching power source circuitin accordance with the present invention. FIGS. 8 to 14 show embodimentsof the multi-output switching power source circuit in accordance withthe present invention.

First Embodiment

FIG. 8 shows a configuration of electric connections of an embodiment ofthe a multi-output switching power source circuit in accordance with thepresent invention. Description will now be given in detail of theembodiment by referring to FIG. 8.

The multi-output switching power circuit in accordance with the presentinvention is adopted a forward converter type and includes a switchingcircuit(s) which is produced a predetermined ac voltage in a primarywinding and an auxiliary winding 122 by applying an input voltage from adc power source(s) and on/off controlling by the transistor 111.

The configuration of the power source circuit includes a transformer 120having a primary side and a secondary side. The circuit includes on theprimary side a dc power source 101, an input smoothing capacitor 102, astarting resistor 103, a PWM control circuit 104, detecting resistors105 and 106, a capacitor 107, a smoothing choke coil 108, a rectifyingdiode 109, a commutating diode 110, a switching circuit such as an NMOS111, and an active clamp circuit 112. The circuit 112 includes acapacitor 113 and an NMOS 114.

The dc power supply 101 includes, for example, a battery and produces adc input voltage V101.

The input smoothing capacitor 102 smoothes the dc input voltage V101.

The starting resistor 103 regulates a start-up current supplied in a PWMcontrol circuit 104 by applying voltage from dc power source 101 onstart up. During power up the power source circuit of the presentinvention, the PWM control circuit 104 is run by the dc voltage V107which is produced by the auxiliary winding 122 and the de voltage V107also becomes a voltage supply source of the PWM control circuit.

The transformer 120 includes a subordinate winding 122 on the primaryside to produce an ac voltage V122 associated with a turn ratio betweenthe primary winding 120 and the subordinate winding 122. The diode 109rectifies the ac voltage V122 into a pulsating voltage V109 havingelectromagnetic energy. The smoothing choke coil 108 accumulates theelectromagnetic energy. When the diode 109 is off and the diode 110 ison, the electromagnetic energy is supplied to the smoothing capacitor107. The capacitor 107 smoothes the pulsating voltage V109 into a dcvoltage V107 to be divided by the resistors 105 and 106. The PWMcontroller 104 detects variation in the voltage. By controlling the PWMcontroller 104, the dc voltage V107 is stabilized. The dc voltage V107set in a voltage supply of the PWM controller 104.

The PWM control circuit 104 generates a control signal V104A having apredetermined frequency and a pulse width corresponding to the detectedvoltage to conduct negative feedback control for a pulse width of an acvoltage V111, which will be described later. The PWM controller 104generates, in addition to the control signal V104A to control the NMOS111, a control signal V104B opposite in phase to the control signalV104A to control the NMOS 114.

According to the control signal V104A from the controller 104, the NMOS111 turns the dc input voltage V101 on or off to generate the ac voltageV111 having a predetermined frequency and a pulse width corresponding tothe detected voltage.

The NMOS 114 turns on when the NMOS 111 is off, a resonance circuit isconfigured by a primary winding 120 and the capacitor 113 of the voltagetransformer 120 to flow an exciting current to the primary winding 121to reset the core of the transformer 120.

The transformer 120 includes the primary winding 121 and the subordinatewinding 122 on the primary side and the secondary winding 123, asubordinate winding 124 to generate a control voltage V124 for on/offcontrol of an NMOS 132 and a subordinate winding 125 to generate acontrol voltage V125 for on/off control of an NMOS 133 on the secondaryside.

The secondary side of the transformer 120 includes a plurality of outputsections of which each includes a magnetic amplifier 131, synchronousrectifying FETs 132 and 133, a Schottky barrier diode 134, a smoothingchoke coil 135, a smoothing capacitor 136, a constant-voltage controller137, resistors 138 and 139, a transistor 140, a resistor 141, and adiode 142. Each output section is connected to a load RL.

The magnetic amplifier 131 turns on or off the ac voltage V123 from thesecondary winding 123 according to a reset current T142 to produce an acvoltage V131 having a pulse width associated with the reset current1142.

The NMOS 132 includes a gate electrode, a source electrode, and a drainelectrode. The gate electrode is connected to a winding end port of thesubordinate winding 124, the source electrode is coupled with an outputside of the magnetic amplifier 131, and the drain electrode is linkedwith a drain electrode of the NMOS 133. The NMOS 132 constitutes asynchronous rectifying circuit and turns on or off the ac voltage V131from the magnetic amplifier 131 in synchronism with a change in polarityof the ac voltage V124 from the subordinate winding 124 to generate apulsating voltage V132.

The smoothing choke coil 135 accumulates electromagnetic energy of thepulsating voltage V132.

The NMOS 133 includes a gate electrode, a source electrode, and a drainelectrode. The gate electrode is connected to a winding start port ofthe subordinate winding 125, the source electrode is linked with ground,and the drain electrode is coupled with the drain electrode of the NMOS132. The NMOS 133 is on when the NMOS 132 is off to supply theelectromagnetic energy from the choke coil 135 to the smoothingcapacitor 136.

The capacitor 136 smoothes the pulsating voltage V132 to generate a dcoutput voltage V and applies the voltage V to the load RL.

The voltage V is divided by the resistors 138 and 139. Theconstant-voltage controller 137 detects variation in the voltage.

According to the sensed change in the dc output voltage V, thecontroller 137 generates a reset current 1142 for negative feedbackcontrol of the ac voltage V131.

The constant-voltage circuit 137 controls a current flowing through thetransistor 140. The reset current I142 is fed from a collector of thetransistor 140 via the resistor 141 and the diode 142 to the magneticamplifier 131. This resultantly achieves the negative feedback controlof the ac voltage V131 to stabilize the dc output voltage V.

In the embodiment, a Schottky barrier diode 134 is connected in parallelwith the NMOS 133. The configuration will be described by referring toFIG. 9. Using the reset current I142 from the constant-voltage circuit137, the amplifier 131 controls a reguration of “on” width (a period ofon cycle) by hindering occurrence of an ac voltage on the secondarywinding 123 of the transformer 120. As can be seen from FIG. 9, the NMOS132 is on and the NMOS 133 is off during this period of time blocked bymagamp 131. As a result during the reguration of “on” width (theperiod), a load current through the smoothing choke coil 135 flowsthrough a body diode of the NMOS 133 when the NMOS 132 is on. The bodydiode causes a large voltage drop. Therefore, a Schottky barrier diode134 causing a small voltage drop is connected in parallel therewith.Thanks to the constitution, during the prevention period of time of themagnetic amplifier 131, the current of the smoothing choke coil 135 ispassed therethrough to remarkably improve the conversion efficiency.

FIG. 10 shows waveforms in various sections of the embodiment of themulti-output switching power source circuit in a schematic graph inwhich the abscissa and the ordinate represent voltage and time,respectively.

Referring to the graph of FIG. 10, description will be given ofoperation of the embodiment of the multi-output switching power sourcecircuit.

The dc power source 101 generates and outputs a dc input voltage V101.The voltage V101 is smoothes by the input smoothing capacitor 102. ThePWM controller 104 generates a control signal V104A having apredetermined frequency and a pulse width associated with variation inan ac voltage generated by the subordinate winding 122. The dc inputvoltage V101 is turned on or off by the NMOS 111 according to thecontrol signal V104A to produce an ac voltage V111 having apredetermined frequency and a pulse width associated with the controlsignal V104A. The ac voltage V111 is transformed or converted by thevoltage transformer 120 to generate ac voltages V123 to V127 on thesecondary side of the transformer 120.

Next, description will be given of operation of the active clamp circuit112 to reset the core of the transformer 120.

The NMOS 111 and the NMOS 114 complementarily turn on and off. However,timing of the control signal V104A from the controller 104 and timing ofthe control signal V104B from the controller 104 have a deadtime toprevent an event in which the NMOS 111 and the NMOS 114 are set to an onstate at the same time. During a period of time in which the NMOS 114 ison, the primary winding 121 and the capacitor 113 of the transformer 120forms a resonance circuit to flow an exciting current to the primarywinding 121 to reset the core of the transformer 120. Therefore, thewaveform of the ac voltage V111 becomes similar to a rectangularwaveform as shown in FIG. 10 and hence the waveform of the ac voltageV123 becomes similar to the rectangular waveform. As a result, the gatevoltage of the NMOS 133 has almost an ideal rectangular waveform, andconduction loss of the NMOS 133 is reduced to increase the conversionefficiency.

The ac voltage V123 appearing on the secondary side is turned on or offby the magnetic amplifier 131 according to the reset current I142 toproduce an ac voltage V131 having a pulse width associated with thereset current I142. In this case, since the time width of the on stateof the NMOS 111 is not abruptly reduced, the ac voltage V123 on thesecondary winding 123 of the transformer 120 has a pulse width necessaryfor the saturation of the amplifier 131. For the amplifier 131, thevoltage-time product is guaranteed (i.e., the product of V×T, where V isthe voltage across the amplifier 131 and T is the time for thesaturation of the amplifier). The operation will be described later indetail.

The ac voltage V131 from the amplifier 131 is turned on or off by theNMOS 132 at timing synchronized with the change in polarity of the acvoltage V124, namely, the change in the control voltage V124 of the NMOS132 to resultantly produce a pulsating voltage having electro-magneticenergy. The energy is accumulated in the smoothing choke coil 135. Theelectromagnetic energy is turned on or off by the NMOS 133 insynchronism with the variation in polarity of the ac voltage V125, thatis, the variation in the control voltage V125 of the NMOS 133. When theNMOS 132 is off and the NMOS 133 is on, the energy is supplied to thesmoothing capacitor 136. The pulsating voltage is smoothed by thesmoothing capacitor 136 into a dc output voltage V. The output voltage Vis applied to the load RL. The dc voltage V is divided by the resistors138 and 139. Variation in the voltage is sensed by the constant-voltagecontroller 137. The controller 137 controls a current of the transistor140. The reset current I142 is fed from the collector of the transistor140 via the resistor 141 and the diode 142 to the magnetic amplifier 131to resultantly conduct negative feedback control for the ac voltageV123. This stabilizes the dc output voltage V.

Description will now be given of the time width of the on state of theNMOS 111 when the load RL becomes small.

When the load current flowing through the smoothing choke coil 135 isequal to or less than a critical current of the choke coil 135, thecurrent flows in both directions when the NMOS 132 is on as shown inFIG. 10. Therefore, the load current also flows in the reversedirection. Excessive energy in the small-load state reversely flowsthrough the choke coil 135 via the transformer 120 to the primary sidethereof. As a result, the load current flowing through the coil 135becomes continuous. In response to variation in the load current, thevoltage across the coil 135 varies, as shown in FIG. 10, between[V123−V] and [−V], where V123 is a voltage on the secondary winding 123of the transformer 120 when the NMOS 111 is on and V is an outputvoltage from the transformer 120. Therefore, even when the load issmall, the dc output voltage is not increased, and hence the time widthof the on state of the NMOS 111 is not abruptly reduced.

The embodiment described above includes NMOS in place of diodes used inthe prior art for rectification and commutation, and the configurationof the embodiment includes a combination of the NMOS for synchronousrectification and a magnetic amplifier for constant-voltage control. Asa result, it is not required to use, for example, a radiator todissipate heat. Therefore, the system size is reduced and the systemefficiency is increased, and the system can be easily implemented in alow-voltage multi-output configuration. Thanks to the removal of thediodes, heat generated in the system is reduced. This increasesreliability of the system and contributes to the saving of energyconsumed by the system. Since the magnetic amplifier controls thesecondary side, it is possible to easily configure a highly stablemulti-output power source with reduced interference between the outputs.

In the configuration, the magnetic amplifier 131 is arranged between thesecondary winding 123 and the NMOS 132 for synchronous rectification andthe driving circuit of the NMOS 132 and 133 for synchronousrectification is implemented using a winding other than the secondarywinding 123. Therefore, the NMOS 132 is not included in the loop to flowthe reset current. The magnetic amplifier 131 can conductconstant-voltage control without any influence from the interruption ofthe control loop when the NMOS 132 on the rectifying side is turned off.

By disposing a Schottky barrier diode 134 causing a small voltage dropin parallel with the body diode of the NMOS 133, the current of thesmoothing choke coil 135 can be passed through the Schottky barrierdiode to increase the conversion efficiency during the period of time inwhich the amplifier 131 prevents the on cycle.

Additionally, by disposing the active clamp circuit 112 on the primaryside of the transformer 120, the flyback voltage has almost arectangular waveform when the NMOS 111 is off. This guarantees that theNMOS 133 for synchronous rectification on the commutation side is turnedon regardless of input and load variations. Utilization efficiency ofthe transformer can be increased, for example, the transformer canoperate in a wide input configuration.

The multi-output switching power supply circuit shown in FIG. 11 can beconsidered as a variation of the embodiment described above. In thevariation, the gate voltage to control the NMOS 133 is obtained not fromthe subordinate winding 125 of the transformer 120. That is, there isprovided a coil 150 including a subordinate winding 151, a winding startpoint of which is connected to the gate electrode of the NMOS 133.Therefore, the gate electrode of the NMOS 133 is applied with an acvoltage opposite in polarity to the voltage applied to the smoothingchoke coil 150 as shown in FIG. 12. During the on-cycle preventionperiod by the amplifier 131, the NMOS 133 is on by the ac voltagegenerated by the subordinate winding 151 as shown in FIG. 12. Thecurrent of the choke coil 150 can be passed through the NMOS 133.Therefore, it is not required to arrange the Schottky barrier diode inparallel with the body diode of the NMOS 133 as in the first embodiment.The voltage V123 of FIG. 12 is generated by the secondary winding 123 ofthe transformer 120 when the NMOS 111 is on, and V represents an outputvoltage. The waveforms shown in FIG. 12 are examples when the turn ratiobetween the choke coil 150 and the subordinate winding 151 is one. Bychanging the turn ratio therebetween, the voltage applied to the gateelectrode of the NMOS 133 can be arbitrarily adjusted.

Second Embodiment

Next, description will be given of a second embodiment in accordancewith the present invention by referring to the accompanying drawings.FIG. 13 shows a circuit configuration of the second embodiment inaccordance with the present invention.

The second embodiment is a multi-output switching power source circuitincluding a main output section and a plurality of subordinate outputsections. Among the output sections, the main output section is anoutput section operates with a maximum output and small load variation.The circuit includes a primary side of which a switching duty ratio iscontrolled by negative feedback according to variation in an outputvoltage from the main output section. Output voltages from thesubordinate output sections are controlled by feedback of an ac voltagewith a duty ratio determined according to an output voltage from themain output section.

The main output section of the second embodiment differs from theassociated output section of the first embodiment in that thesubordinate winding to drive the NMOS 231 and the NMOS 232 is notarranged, the magnetic amplifier 131 is not used, and the transistor141, the resistor 142, and the diode 143 to supply the reset current tothe amplifier 131 are not disposed.

The NMOS 231 includes a gate electrode connected to a winding end pointof the secondary winding 223, a source electrode coupled with a sourceelectrode of the NMOS 232 in the subsequent stage, and a drain electrodelinked with a winding start point of the secondary winding 223. The NMOS232 includes a gate electrode coupled with the winding start point ofthe secondary winding 223, the source electrode linked with the sourceelectrode of the NMOS 231, and a drain electrode connected to thewinding end point of the secondary winding 223.

The NMOS 231 constitutes a synchronous rectifying circuit and turns onor off an ac voltage V223 on the secondary winding according to changein polarity of the ac voltage V223 to produce a pulsating voltage V231The NMOS 232 turns on when the NMOS 231 is set to off to supplyelectromagnetic energy from the smoothing choke coil 233 to thesmoothing capacitor 234.

The main output section A of the second embodiment does not include themagnetic amplifier and the transistor, the resistor, and the diode tosupply the reset current to the amplifier. Therefore, at detection ofvariation in the dc output voltage VI from the smoothing coil 234, aconstant-voltage controller 235 notifies a detection signal indicatingthe detected voltage change to a PWM controller 204. The controller 204generates a control signal V204 with a pulse width according to a changeof an ac voltage appearing on a subordinate winding 222 in proportion toan associated turn ratio and to the detection signal from theconstant-voltage controller 235.

The subordinate output sections are configured almost in the same way asfor the output section of the first embodiment and are connected toloads RL2 and RL3, respectively.

The second embodiment configured as above is effectively applicable whenthere exists a large-current output which cannot be controlled by amagnetic amplifier. The second embodiment can also obtain anadvantageous effect similar to that of the first embodiment.

Third Embodiment

Referring now to the accompanying drawings, description will be given ofa third embodiment in accordance with the present invention. FIG. 14shows a configuration of the third embodiment of the present invention.

The third embodiment and the first embodiment are configured basicallyin the same way. The third embodiment includes a devised configurationof an NMOS synchronous rectifying circuit and a magnetic amplifiercontrol circuit.

In the third embodiment, an NMOS 342 and an NMOS 343 for synchronousrectification are driven by a voltage generated by a secondary winding323, and a magnetic amplifier is arranged between a gate electrode ofthe NMOS 342 on a rectification side and a drain electrode of the NMOS343 on a commutation side. The magnetic amplifier 341 additionallyincludes a reset winding to reset the core of the amplifier 341. Aconstant-voltage circuit 347 supplies a reset current to a winding startpoint of the reset winding to be delivered via the NMOS 342 to a groundside in a subsequent stage. Therefore, the loop to flow the resetcurrent does not include the NMOS 342 for synchronous rectification. Themagnetic amplifier can be turned on or off without any influence fromthe interruption of the control loop when the NMOS 342 is turned off.Therefore, the constant-voltage control can be achieved. Since theseparate subordinate winding is not required, the transformer can bereduced in size.

The embodiments described above are suitable for the present invention.However, the present invention is not restricted by the embodiments andcan be modified in various ways without departing from scope of thepresent invention. For example, although NMOS are used to construct asynchronous rectifying circuit in the embodiments, p-type MOS (PMOS) mayalso be employed for the same purpose.

As can be seen from the description of the present invention, NMOS areused in place of the rectifying and commutating diodes of the prior artand the circuit configuration includes a combination of the synchronousrectifying NMOS and a magnetic amplifier for constant-voltage control.Since the heat dissipation using, for example, a radiator is notrequired, the system size can be reduced. The conversion efficiency canbe increased and the system can be easily implemented in a low-voltagemulti-output configuration. The diodes can be removed and hence heatgenerated in the system is reduced. This increases reliability of thesystem and saves energy consumed by the system. Since the magneticamplifier is employed to control the secondary side, a highly stablemulti-output power source with reduced interference between the outputscan be easily implemented.

By disposing the magnetic amplifier between the secondary winding andthe NMOS constituting the rectifying circuit and by arranging a separatesubordinate winding other than the secondary winding as the drivingcircuit of the NMOS of the rectifying circuit, the rectifying circuit isnot configured in the loop in which the reset current flows. Themagnetic amplifier can achieve constant-voltage control without anyinfluence from the interruption of the control loop when the diode onthe rectifying side is turned off.

By arranging a diode causing a small voltage drop in parallel with thebody diode of the NMOS constituting the smoothing circuit, the currentof the smoothing choke coil flows through the diode during the period inwhich the magnetic amplifier prevents the on cycle to thereby increasethe conversion efficiency.

Since the active clamp circuit is disposed on the primary side of thetransformer, the flyback voltage has almost a rectangular waveform whenthe switching circuit is off. It is therefore guaranteed that the NMOSconstituting the smoothing circuit is turned on regardless of input andload variation. Therefore, the transformer has improved usability, forexample, can cope with a wide input system.

While the present invention has been described with reference to theparticular illustrative embodiments, it is not to be restricted by thoseembodiments but only by the appended claims. It is to be appreciatedthat those skilled in the art can change or modify the embodimentswithout departing from the scope and spirit of the present invention.

What is claimed is:
 1. A multi-output switching power supply circuit,comprising: a dc power source for generating a dc input voltage; adetecting circuit for detecting a voltage value of a second ac voltagegenerated by a first subordinate winding, the first subordinate windingconstituting a voltage transformer including a primary side, a primarywinding, a core, a secondary side, and a secondary winding; a switchingcircuit for turning on or off the dc input voltage according to acontrol signal generated by detecting variation in the voltage value ofthe second ac voltage and thereby producing a first ac voltage having apredetermined frequency and a pulse width corresponding to the second acvoltage; a control circuit for generating the control signal accordingto variation in the voltage value of the second ac voltage detected bysaid detecting circuit; an active clamp circuit for passing an excitingcurrent through the primary winding of said voltage transformer duringan off period of said switching circuit and for thereby resetting thecore of said voltage transformer, said dc power source, said detectingcircuit, said switching circuit, said control circuit, and said activeclamp circuit being arranged on the primary side of said voltagetransformer; and a plurality of output sections disposed on thesecondary side of said voltage transformer, each of said output sectionscomprising: a magnetic amplifier for controlling, according to a resetcurrent, on or off of a third ac voltage generated on the secondarywinding through voltage conversion of the first ac voltage by saidvoltage transformer and for thereby generating a fourth ac voltagehaving a pulse width corresponding to the reset current; a rectifyingcircuit for rectifying the fourth ac voltage into a pulsating voltage; asmoothing circuit for smoothing the pulsating voltage into a dc outputvoltage and for applying the dc output voltage to a load; and a voltagecontrol circuit for detecting variation in the dc output voltage and forgenerating the reset current to conduct negative feedback control forthe fourth ac voltage, wherein: said rectifying circuit comprises afirst NMOS transistor which is turned on or off according to a voltagevalue of a fifth ac voltage generated on a second subordinate windingdisposed on the secondary side of said voltage transformer and whichthereby generates the pulsating voltage; said smoothing circuitcomprises: a smoothing capacitor for smoothing the pulsating voltageinto the de output voltage and for applying the dc output voltage to aload; a choke coil for accumulating electromagnetic energy associatedwith the pulsating voltage; and a second NMOS transistor which turns on,when said first NMOS transistor is off, according to a voltage value ofa sixth ac voltage generated on a third subordinate winding disposed onthe secondary side of said voltage transformer and which therebysupplies the electromagnetic energy from the choke coil to the smoothingcapacitor; and said magnetic amplifier is arranged between the secondarywinding and said first NMOS transistor.
 2. A multi-output switchingpower supply circuit, comprising: a dc power source for generating a dcinput voltage; a detecting circuit for detecting a voltage value of asecond ac voltage generated by a first subordinate winding, the firstsubordinate winding constituting a voltage transformer including aprimary side, a primary winding, a core, a secondary side, and asecondary winding; a switching circuit for turning on or off the dcinput voltage according to a control signal generated by detectingvariation in the voltage value of the second ac voltage and therebyproducing a first ac voltage having a predetermined frequency and apulse width corresponding to the second ac voltage; a control circuitfor generating the control signal according to variation in the voltagevalue of the second ac voltage detected by said detecting circuit and alevel of a detection signal detected by a voltage variation detectingcircuit; an active clamp circuit for passing an exciting current throughthe primary winding of said voltage transformer during an off period ofsaid switching circuit and for thereby resetting the core of saidvoltage transformer, said dc power source, said detecting circuit, saidswitching circuit, said control circuit, and said active clamp circuitbeing arranged on the primary side of said voltage transformer; a mainoutput section disposed on the secondary side of said voltagetransformer, comprising: a first rectifying circuit for rectifying aseventh ac voltage generated on a first secondary winding throughvoltage conversion of the first ac voltage by said voltage transformerand for thereby generating a first pulsating voltage; a first smoothingcircuit for smoothing the first pulsating voltage into a first dc outputvoltage and for applying the first dc output voltage to a load; and thevoltage variation detecting circuit for detecting variation in the firstdc output voltage into a detection signal and for supplying thedetection signal to said control circuit; and a plurality of outputsections disposed on the secondary side of said voltage transformer,each of said output sections comprising: a magnetic amplifier forcontrolling, according to a reset current, on or off of an eighth acvoltage generated on the secondary winding through voltage conversion ofthe first ac voltage by said voltage transformer and for therebygenerating a ninth ac voltage having a pulse width corresponding to thereset current; a second rectifying circuit for rectifying the ninth acvoltage into a second pulsating voltage; a second smoothing circuit forsmoothing the second pulsating voltage into a second dc output voltageand for applying the second dc output voltage to a load; and a voltagecontrol circuit for detecting variation in the second dc output voltageand for generating the reset current to conduct negative feedbackcontrol for the ninth ac voltage, wherein: said first rectifying circuitcomprises a first NMOS transistor which turns the seventh ac voltage onor off at timing synchronized with switching timing of said switchingcircuit and which thereby generates the first pulsating voltage; saidsmoothing circuit comprises: a first smoothing capacitor for smoothingthe first pulsating voltage into the first dc output voltage and forapplying the first dc output voltage to a load; a first choke coil foraccumulating electromagnetic energy associated with the first pulsatingvoltage; and a second NMOS transistor which turns on when said firstNMOS transistor is off, and which thereby supplies the electro-magneticenergy from the choke coil to the smoothing capacitor; said secondrectifying circuit comprises a third NMOS transistor which is turned onor off according to a voltage value of a tenth ac voltage generated onthe second subordinate winding disposed on the secondary side of saidvoltage transformer; said second smoothing circuit comprises: a secondsmoothing capacitor for smoothing the second pulsating voltage into thesecond dc output voltage and for applying the second dc output voltageto a load; a second choke coil for accumulating electro-magnetic energyassociated with the second pulsating voltage; and a fourth NMOStransistor which turns on, when said third NMOS transistor is off,according to a voltage value of an 11th ac voltage generated on a thirdsubordinate winding disposed on the secondary side of said voltagetransformer and which thereby supplies the electromagnetic energy fromthe second choke coil to the second smoothing capacitor; and saidmagnetic amplifier is arranged between the secondary winding and saidthird NMOS transistor.
 3. A multi-output switching power supply circuit,comprising: a dc power source for generating a dc input voltage; adetecting circuit for detecting a voltage value of a second ac voltagegenerated by a first subordinate winding, the first subordinate windingconstituting a voltage transformer including a primary side, a primarywinding, a core, a secondary side, and a secondary winding; a switchingcircuit for turning on or off the dc input voltage according to acontrol signal generated by detecting variation in the voltage value ofthe second ac voltage and thereby producing a first ac voltage having apredetermined frequency and a pulse width corresponding to the second acvoltage; a control circuit for generating the control signal accordingto variation in the voltage value of the second ac voltage detected bysaid detecting circuit and a level of a detection signal detected by avoltage variation detecting circuit; an active clamp circuit for passingan exciting current through the primary winding of said voltagetransformer during an off period of said switching circuit and forthereby resetting the core of said voltage transformer, said dc powersource, said detecting circuit, said switching circuit, said controlcircuit, and said active clamp circuit being arranged on the primaryside of said voltage transformer; and an output section disposed on thesecondary side of said voltage transformer, comprising: a magneticamplifier for controlling, according to a reset current, on or off of athird ac voltage generated on the secondary winding through voltageconversion of the first ac voltage by said voltage transformer and forthereby generating a fourth ac voltage having a pulse widthcorresponding to the reset current; a rectifying circuit for rectifyingthe fourth ac voltage into a pulsating voltage; a smoothing circuit forsmoothing the pulsating voltage into a dc output voltage and forapplying the dc output voltage to a load; and a voltage control circuitfor detecting variation in the dc output voltage and for generating thereset current to conduct negative feedback control for the fourth acvoltage, wherein: said rectifying circuit comprises a first NMOStransistor which turns the third ac voltage on or off at timingsynchronized with switching timing of said switching circuit and whichthereby generates the pulsating voltage; said smoothing circuitcomprises: a smoothing capacitor for smoothing the pulsating voltageinto the dc output voltage and for applying the dc output voltage to aload; a choke coil for accumulating electromagnetic energy associatedwith the pulsating voltage; and a second NMOS transistor which turns onwhen said first NMOS transistor is off, and which thereby supplies theelectro-magnetic energy from the choke coil to the smoothing capacitor;said first NMOS transistor includes a gate electrode, a sourceelectrode, and a drain electrode, said gate electrode being connected toa winding end side of the secondary winding, said source electrode beinglinked with a ground side, said drain electrode being coupled with awinding start side of the secondary winding; said second NMOS transistorincludes a gate electrode, a source electrode, and a drain electrode,said gate electrode being connected to the winding start side of thesecondary winding, said source electrode being linked with a groundside, said drain electrode being coupled with an output port of saidmagnetic amplifier; said magnetic amplifier is arranged between the gateelectrode of said first NMOS transistor and the drain electrode of saidsecond NMOS transistor; and the reset current is supplied to a windingstart side of said reset winding and is outputted to the ground side. 4.The multi-output switching power supply circuit in accordance with claim1, further comprising a diode having a small voltage drop in a stageafter said second NMOS transistor in parallel with said second NMOStransistor.
 5. The multi-output switching power supply circuit inaccordance with claim 2, further comprising a diode having a smallvoltage drop in a stage after said second NMOS transistor in parallelwith said second NMOS transistor.
 6. The multi-output switching powersupply circuit in accordance with claim 3, further comprising a diodehaving a small voltage drop in a stage after said second NMOS transistorin parallel with said second NMOS transistor.
 7. The multi-outputswitching power supply circuit in accordance with claim 1, wherein saidactive clamp circuit comprises: a capacitor connected to a winding endside of the primary winding of said voltage transformer; and an NMOStransistor including a gate electrode, a source electrode, and a drainelectrode, said gate electrode being connected to a signal deliveredfrom said control circuit, said signal being opposite in phase to thecontrol signal generated from said control circuit; said sourceelectrode being coupled with an output from said switching circuit, saiddrain electrode linked with said capacitor, the signal opposite in phaseto the control signal generated from said control circuit havingdeadtime preventing an event in which said switching circuit and saidNMOS are on at the same time.
 8. The multi-output switching power supplycircuit in accordance with claim 2, wherein said active clamp circuitcomprises; a capacitor connected to a winding end side of the primarywinding of said voltage transformer; and an NMOS transistor including agate electrode, a source electrode, and a drain electrode, said gateelectrode being connected to a signal delivered from said controlcircuit, said signal being opposite in phase to the control signalgenerated from said control circuit; said source electrode being coupledwith an output from said switching circuit, said drain electrode linkedwith said capacitor, the signal opposite in phase to the control signalgenerated from said control circuit having deadtime preventing an eventin which said switching circuit and said NMOS are on at the same time.9. The multi-output switching power supply circuit in accordance withclaim 3, wherein said active clamp circuit comprises: a capacitorconnected to a winding end side of the primary winding of said voltagetransformer; and an NMOS transistor including a gate electrode, a sourceelectrode, and a drain electrode, said gate electrode being connected toa signal delivered from said control circuit, said signal being oppositein phase to the control signal generated from said control circuit; saidsource electrode being coupled with an output from said switchingcircuit, said drain electrode linked with said capacitor, the signalopposite in phase to the control signal generated from said controlcircuit having deadtime preventing an event in which said switchingcircuit and said NMOS are on at the same time.
 10. The multi-outputswitching power supply circuit in accordance with claim 1, wherein saidthird ac voltage has a pulse width necessary for saturation of saidmagnetic amplifier.
 11. The multi-output switching power supply circuitin accordance with claim 3, wherein said third ac voltage has a pulsewidth necessary for saturation of said magnetic amplifier.
 12. Themulti-output switching power supply circuit in accordance with claim 2,wherein said eighth ac voltage has a pulse width necessary forsaturation of said magnetic amplifier.